JP5910806B2 - Optical module and atomic oscillator for an atomic oscillator - Google Patents

Description

  The present invention relates to an optical module for an atomic oscillator and an atomic oscillator.

  In recent years, atomic oscillators using CPT (Coherent Population Trapping), which is one of the quantum interference effects, have been proposed, and miniaturization of devices and low power consumption are expected. An atomic oscillator using a CPT is an oscillator using a phenomenon (EIT phenomenon: Electromagnetically Induced Transparency) in which absorption of two resonance lights is stopped simultaneously when two resonance lights having different wavelengths (frequencies) are simultaneously irradiated onto an alkali metal atom. It is. For example, Patent Document 1 includes, as an atomic oscillator using CPT, a light source that emits coherent light, a gas cell in which alkali metal atoms are sealed, and a light receiving element that detects the intensity of light transmitted through the gas cell. An atomic oscillator configured to include an optical module is described.

  In an atomic oscillator using CPT, for example, a semiconductor laser is used as a light source. In an atomic oscillator using a semiconductor laser as a light source, for example, a sideband is generated in the light emitted from the semiconductor laser by modulating the driving current of the semiconductor laser, thereby causing the EIT phenomenon.

JP 2009-89116 A

  However, the light emitted from the semiconductor laser whose drive current is modulated includes not only the sideband wave but also a fundamental wave (carrier wave) having a center wavelength that does not contribute to the EIT phenomenon. When the alkali metal atom is irradiated with this fundamental wave, the wavelength (frequency) of the light absorbed by the alkali metal atom changes (AC Stark effect), which may reduce the frequency stability of the atomic oscillator.

  One of the objects according to some embodiments of the present invention is to provide an optical module for an atomic oscillator capable of obtaining an atomic oscillator with high frequency stability. Another object of some aspects of the present invention is to provide an atomic oscillator having the optical module for the atomic oscillator.

An optical module for an atomic oscillator according to the present invention,
An optical module for an atomic oscillator using the quantum interference effect,
A light source that emits light including a fundamental wave having a predetermined wavelength, and a sideband of the fundamental wave;
A wavelength selection unit that receives light from the light source and transmits the sideband of the incident light;
A gas cell in which an alkali metal gas is sealed and irradiated with light transmitted through the wavelength selection unit;
A light detector for detecting the intensity of light transmitted through the gas cell among the light irradiated to the gas cell;
Including
The wavelength selector is
An optical filter that transmits light in a predetermined wavelength range;
An optical filter characteristic control unit that changes a wavelength range of light transmitted by the optical filter unit;
Have

  According to such an optical module for an atomic oscillator, the wavelength selection unit can reduce the intensity of the fundamental wave included in the light from the light source or eliminate the fundamental wave. Thereby, it can suppress or prevent that the fundamental wave which does not contribute to an EIT phenomenon is irradiated to an alkali metal atom. Therefore, frequency fluctuation due to the AC Stark effect can be suppressed, and an oscillator with high frequency stability can be provided. Furthermore, since the wavelength selection unit has an optical filter characteristic control unit that changes the wavelength range of the light transmitted by the optical filter unit, the wavelength selection characteristic (optical filter unit) of the optical filter unit due to manufacturing error, environmental change, etc. Can be corrected.

In the optical module for an atomic oscillator according to the present invention,
The optical filter portion is an etalon,
The optical filter characteristic control unit can change a wavelength range of light transmitted by the optical filter unit by an electro-optic effect.

  According to such an optical module for an atomic oscillator, the optical filter portion can be configured simply. Furthermore, the wavelength selection characteristic of the optical filter section can be controlled with high accuracy and ease.

In the optical module for an atomic oscillator according to the present invention,
The optical filter section is
A first mirror and a second mirror that reflect the light incident on the wavelength selection unit and face each other;
A substrate disposed between the first mirror and the second mirror;
Have
The substrate may be a compound semiconductor.

  According to such an optical module for an atomic oscillator, the distance between the first mirror and the second mirror can be reduced, and the apparatus can be miniaturized.

In the optical module for an atomic oscillator according to the present invention,
The optical filter characteristic control unit may include a first electrode and a second electrode that apply a voltage to the substrate.

  According to such an optical module for an atomic oscillator, the optical filter characteristic control unit can have a simple configuration.

In the optical module for an atomic oscillator according to the present invention,
In addition, a compound semiconductor substrate,
The light source is a semiconductor laser;
The light filter part and the light source are located on the base,
The first electrode is located on the opposite side of the base on which the optical filter portion is located,
The second electrode may be located on the opposite side of the optical filter portion from the side where the substrate is located.

  According to such an optical module for an atomic oscillator, since the wavelength selection unit and the light source are formed on the same substrate, the apparatus can be miniaturized.

  In the description according to the present invention, the word “upper” is used, for example, “specifically” (hereinafter referred to as “A”) is formed above another specific thing (hereinafter referred to as “B”). The word “above” is used to include the case where B is formed directly on A and the case where B is formed on A via another object. Used. Similarly, the term “below” includes a case where B is directly formed under A and a case where B is formed under another through A.

In the optical module for an atomic oscillator according to the present invention,
The substrate of the optical filter portion has a first layer, a second layer, and a third layer laminated from the base side,
The refractive index of the first layer and the refractive index of the third layer are smaller than the refractive index of the second layer,
The second layer can propagate light emitted from the light source.

  According to such an optical module for an atomic oscillator, the substrate can be an optical waveguide. Therefore, the beam diameter of the light emitted from the wavelength selection unit can be controlled, and the light transmitted through the wavelength selection unit can be efficiently irradiated onto the gas cell.

In the optical module for an atomic oscillator according to the present invention,
The light source may be an edge emitting laser.

  According to such an optical module for an atomic oscillator, alignment between the light source (edge-emitting laser) and the wavelength selection unit can be performed by controlling the film thickness of the layer constituting the edge-emitting laser. Therefore, the alignment accuracy between the light source and the wavelength selector can be improved. Furthermore, for example, an optical element for making the light from the light source incident on the wavelength selection unit becomes unnecessary.

In the optical module for an atomic oscillator according to the present invention,
The light source may be a surface emitting laser.

  According to such an optical module for an atomic oscillator, the surface-emitting laser has a smaller current for generating a gain than the edge-emitting laser, so that the power consumption can be reduced.

The atomic oscillator according to the present invention is
An optical module for an atomic oscillator according to the present invention is included.

  Since such an atomic oscillator includes the optical module for an atomic oscillator according to the present invention, frequency fluctuation due to the AC Stark effect can be suppressed and frequency stability can be increased.

The atomic oscillator according to the present invention is
For atomic oscillators using quantum interference effects,
A light source that emits light including a fundamental wave having a predetermined wavelength, and a sideband of the fundamental wave;
A wavelength selection unit that receives light from the light source and transmits the sideband of the incident light;
A gas cell in which an alkali metal gas is sealed and irradiated with light transmitted through the wavelength selection unit;
A light detector for detecting the intensity of light transmitted through the gas cell among the light irradiated to the gas cell;
Including
The wavelength selector is
An optical filter unit for selecting and emitting light in a predetermined wavelength range;
An optical filter characteristic control unit that changes a wavelength range selected by the optical filter unit;
Have

  According to such an atomic oscillator, the wavelength selection unit can reduce the intensity of the fundamental wave included in the light from the light source or eliminate the fundamental wave. Thereby, it can suppress or prevent that the fundamental wave which does not contribute to an EIT phenomenon is irradiated to an alkali metal atom. Therefore, frequency fluctuation due to the AC Stark effect can be suppressed, and an oscillator with high frequency stability can be provided. Furthermore, since the wavelength selection unit has an optical filter characteristic control unit that changes the wavelength range of the light transmitted by the optical filter unit, the wavelength selection characteristic (optical filter unit) of the optical filter unit due to manufacturing error, environmental change, etc. Can be corrected.

The functional block diagram of the atomic oscillator which concerns on this embodiment. FIG. 2A is a diagram showing the relationship between the Λ-type three-level model of alkali metal atoms and the first sideband and second sideband, and FIG. 2B shows the frequency spectrum of the first light generated by the light source. FIG. The figure which shows the frequency spectrum of the 2nd light inject | emitted from the wavelength selection part. The block diagram which shows the structure of the atomic oscillator which concerns on this embodiment. Sectional drawing which shows typically the principal part of the optical module which concerns on this embodiment. Sectional drawing which shows typically the principal part of the optical module which concerns on this embodiment using an edge-emitting laser.

  Preferred embodiments of the present invention will be described below with reference to the drawings.

  First, an optical module and an atomic oscillator according to this embodiment will be described with reference to the drawings. The atomic oscillator according to the present embodiment includes the optical module according to the present embodiment. FIG. 1 is a functional block diagram of an atomic oscillator 1 according to this embodiment. The atomic oscillator 1 is an oscillator using a quantum interference effect.

  The atomic oscillator 1 includes an optical module 2 and a control unit 50.

  The optical module 2 includes a light source 10, a wavelength selection unit 20, a gas cell 30, and a light detection unit 40.

  The light source 10 generates first light L1 including a fundamental wave F having a predetermined center wavelength (center frequency), and a first sideband wave W1 and a second sideband wave W2 having different wavelengths.

  The wavelength selector 20 selects the first sideband wave W1 and the second sideband wave W2 from the first light L1, and emits them as the second light L2. The wavelength selection unit 20 includes an optical filter unit 20a that selects and emits light in a predetermined wavelength range, and an optical filter characteristic control unit 20b that changes the wavelength range selected by the optical filter unit 20a.

  The gas cell 30 encloses an alkali metal gas, and the gas cell 30 is irradiated with the second light L2.

  The light detection unit 40 detects the intensity of the second light L2 that has passed through the gas cell 30.

Based on the detection result of the light detection unit 40, the control unit 50 determines that the difference in wavelength (frequency) between the first sideband wave W1 and the second sideband wave W2 is two ground levels of alkali metal atoms enclosed in the gas cell 30. The frequency is controlled to be equal to the frequency corresponding to the energy difference. Control unit 50 based on the detection result of the optical detection unit 40, generates a detection signal having a modulation frequency f _m. The light source 10 modulates the fundamental wave F having a predetermined frequency _{f 0} based on the detection signal, the first sideband W1 having a frequency _f 1 ₌ f 0 + _{f m,} and the frequency _f 2 = _{f 0} generating a second sideband wave W2 having -f _m.

  FIG. 2A is a diagram showing the relationship between the Λ-type three-level model of alkali metal atoms and the first sideband wave W1 and the second sideband wave W2. FIG. 2B is a diagram showing a frequency spectrum of the first light L1 generated by the light source 10. As shown in FIG.

As shown in FIG. 2B, the first light L1 generated in the light source 10 has a basic frequency f ₀ (= v / λ ₀ : v is the speed of light, and λ ₀ is the center wavelength of the laser light). and wave F, the first sideband W1 having a frequency f ₁ to the upper sideband with respect to the center frequency f _0, and the second sideband wave W2 having a frequency f ₂ to the lower sideband with respect to the center frequency f ₀ ,including. Frequency _{f 1} of the first sideband W1 _{is f 1 = f 0 + f m} , the frequency _{f 2} of the second sideband wave W2 _is f _{2 =} f 0 -f _m.

As shown in FIG. 2 (A) and FIG. 2 (B), the frequency difference between the frequency _{f 2} of the frequency _{f 1} and the second sideband wave W2 of the first sideband W1 is a ground level GL1 alkali metal atom It coincides with the frequency corresponding to the energy difference Delta] E ₁₂ ground levels GL2. Thus, alkali metal atom, causes EIT phenomenon by the second sideband wave W2 having a first sideband wave W1 and the frequency _{f 2} having a frequency _{f 1.}

Here, the EIT phenomenon will be described. It is known that the interaction between alkali metal atoms and light can be explained by a Λ-type three-level model. As shown in FIG. 2A, the alkali metal atom has two ground levels, and a first sideband having a wavelength (frequency f ₁ ) corresponding to the energy difference between the ground level GL1 and the excited level. When an alkali metal atom is irradiated with W1 or a second sideband wave W2 having a wavelength (frequency f ₂ ) corresponding to the energy difference between the ground level GL2 and the excitation level, light absorption occurs. However, as shown in FIG. 2B, the frequency difference f ₁ -f ₂ exactly matches the frequency corresponding to the energy difference ΔE ₁₂ between the ground level GL 1 and the ground level GL 2. When the first sideband wave W1 and the second sideband wave W2 are irradiated at the same time, the two base levels are superposed, that is, a quantum interference state, and the excitation to the excitation level is stopped and the first sideband wave W1 and the second sideband wave W2 A transparency phenomenon (EIT phenomenon) in which the sideband wave W2 passes through the alkali metal atom occurs. When the EIT phenomenon utilizing a frequency difference _f 1 -f ₂ between the first sideband wave W1 and the second sideband wave W2 is shifted from the frequency corresponding to the energy difference Delta] E ₁₂ ground levels GL1 and ground level GL2 By detecting and controlling a steep change in the light absorption behavior of the laser, a highly accurate oscillator can be produced.

  However, when the first light L1 shown in FIG. 2B is directly applied to the gas cell 30, the fundamental wave F is applied to the gas cell 30, that is, the alkali metal atoms simultaneously with the first sideband W1 and the second sideband W2. Will be. When an alkali metal atom is irradiated with a fundamental wave F that does not contribute to the EIT phenomenon, the wavelength (frequency) of light absorbed by the alkali metal atom changes due to the AC Stark effect. Thereby, the quantity of the 1st sideband wave W1 and the 2nd sideband wave W2 which permeate | transmit an alkali metal atom will change. In an oscillator using the EIT phenomenon, the modulation frequency fm is stabilized by detecting the amount of the first sideband wave W1 and the second sideband wave W2 that pass through the alkali metal atom, and this modulation frequency fm is output from the transmitter. As a result, the frequency stability of the transmitter is increased. Therefore, the AC Stark effect generated by the fundamental wave F decreases the detection accuracy of the first sideband wave W1 and the second sideband wave W2, and decreases the stability of the modulation frequency fm. That is, the frequency stability of the transmitter is lowered.

Figure 3 is a diagram showing a frequency spectrum of the second light L2 emitted from the wavelength selection selecting section 20.

The second light L2 is light in which the fundamental wave F disappears or the intensity of the fundamental wave F decreases compared to the first light L1. FIG The third example, the second light L2, the center frequency the first sideband having a frequency f ₁ to the upper sideband with respect to f ₀ W1, and the center frequency f ₂ to the lower sideband with respect to the frequency f ₀ Only the second sideband W2 having Thus, in the optical module 2, the wavelength selector 20 can reduce the intensity of the fundamental wave F or eliminate the fundamental wave F.

  Hereinafter, a more specific configuration of the atomic oscillator of this embodiment will be described.

  FIG. 4 is a block diagram showing the configuration of the atomic oscillator 1.

  As shown in FIG. 4, the atomic oscillator 1 includes an optical module 2, a current drive circuit 150, and a modulation circuit 160.

  The optical module 2 includes a semiconductor laser 110, a wavelength selection device 120, a gas cell 130, and a photodetector 140.

The semiconductor laser 110 generates the first light L1 including the fundamental wave F having a predetermined center wavelength and the first sideband wave W1 and the second sideband wave W2 having different wavelengths. The center frequency f ₀ (center wavelength λ ₀ ) of the laser light (first light L 1) emitted from the semiconductor laser 110 is controlled by the drive current output from the current drive circuit 150, and the output signal (modulation signal) of the modulation circuit 160. Is modulated by. That is, the first light L1 emitted from the semiconductor laser 110 can be modulated by superimposing the alternating current having the frequency component of the modulation signal on the drive current generated by the current drive circuit 150. Thereby, the 1st sideband wave W1 and the 2nd sideband wave W2 are produced | generated by the 1st light L1. Since light generated in the semiconductor laser 110 has coherence, it is suitable for obtaining a quantum interference effect.

As shown in FIG. 2B, the first light L1 includes a fundamental wave F having a center frequency f ₀ (= v / λ ₀ : v is the speed of light, and λ ₀ is the center wavelength of the first light L1). includes a first sideband W1 having a frequency f ₁ to the upper sideband with respect to the center frequency f _0, and the second sideband wave W2 having a frequency f ₂ to the lower sideband with respect to the center frequency f _0, the . Frequency _{f 1} of the first sideband W1 _{is f 1 = f 0 + f m} , the frequency _{f 2} of the second sideband wave W2 _is f _{2 =} f 0 -f _m.

  The wavelength selection device 120 selects the first sideband wave W1 and the second sideband wave W2 from the first light L1, and emits them as the second light L2. The wavelength selection device 120 includes an optical filter element 120a that selects and emits light in a predetermined wavelength range, and an optical filter characteristic control device 120b that changes the wavelength range selected by the optical filter element 120a.

  The optical filter element 120a can select and emit the first sideband wave W1 and the second sideband wave W2 from the first light. Therefore, the intensity of the fundamental wave F of the first light L1 incident on the optical filter element 120a can be reduced or the fundamental wave F can be extinguished and emitted as the second light L2. That is, in the second light L2, the intensity of the fundamental wave F decreases or the fundamental wave F disappears compared to the first light L1. In the example of FIG. 3, the second light L2 has only the first sideband wave W1 and the second sideband wave W2. As will be described later, the optical filter element 120a may be an etalon or a fiber Bragg grating (FBG).

  The optical filter characteristic control device 120b can change the wavelength range (wavelength selection characteristic) selected by the optical filter element 120a. The optical filter characteristic control device 120b can change the wavelength selection characteristic of the optical filter element 120a (for example, etalon) by, for example, the electro-optic effect. Here, the electro-optic effect refers to a phenomenon in which the refractive index of a substance with respect to light changes when an electrostatic field is applied from the outside. The optical filter characteristic control device 120b changes the wavelength selection characteristic of the optical filter element 120a by changing the refractive index of the optical filter element 120a by applying an electric field to the optical filter element 120a, for example. Since the wavelength selection device 120 can correct the shift of the wavelength selection property of the optical filter element 120a due to a manufacturing error or environmental change (heat, light, etc.) by the optical filter property control device 120b, The first sideband wave W1 and the second sideband wave W2 can be selected with high accuracy.

  The optical filter characteristic control device 120b may control the wavelength selection characteristic of the optical filter element 120a by adjusting the intensity of the electric field applied to the optical filter element 120a based on the output signal of the photodetector 140. In the optical module 2, for example, the intensity of the electric field applied to the optical filter element 120a is adjusted by a feedback loop that passes through the optical filter element 120a, the gas cell 130, the photodetector 140, and the optical filter characteristic control device 120b. Is controlled.

  In addition, the optical filter characteristic control device 120b adjusts the intensity of the electric field applied to the optical filter element 120a based on the preliminarily acquired data of the wavelength selection characteristic deviation of the optical filter element 120a, and the optical filter element 120a The shift in wavelength selection characteristics may be corrected.

  The gas cell 130 is a container in which gaseous alkali metal atoms (sodium (Na) atoms, rubidium (Rb) atoms, cesium (Cs) atoms, etc.) are sealed. The gas cell 130 is irradiated with the second light L2 emitted from the wavelength selection device 120.

When the gas cell 130 is irradiated with two light waves (first sideband and second sideband) having a frequency (wavelength) difference corresponding to the energy difference between the two ground levels of alkali metal atoms, Metal atoms cause the EIT phenomenon. For example, if the alkali metal atom is a cesium atom, the frequency corresponding to the energy difference between the ground level GL1 and the ground level GL2 in the D1 line is 9.19263... GHz, so the frequency difference is 9.19263. When two light waves of GHz are irradiated, an EIT phenomenon occurs.

  The photodetector 140 detects the second light L2 that has passed through the gas cell 130, and outputs a signal having a signal intensity corresponding to the detected amount of light. The output signal of the photodetector 140 is input to the current drive circuit 150 and the modulation circuit 160. Further, the output signal of the photodetector 140 may be further input to the optical filter characteristic control device 120b. The photodetector 140 is, for example, a photodiode.

The current drive circuit 150 generates a drive current having a magnitude corresponding to the output signal of the photodetector 140, supplies the drive current to the semiconductor laser 110, and controls the center frequency f ₀ (center wavelength λ ₀ ) of the first light L1. . The center frequency f ₀ (center wavelength λ ₀ ) of the first light is finely adjusted and stabilized by a feedback loop passing through the semiconductor laser 110, the wavelength selection device 120, the gas cell 130, the photodetector 140, and the current drive circuit 150.

Modulation circuit 160 generates a modulated signal having a modulation frequency f _m corresponding to the output signal of the photodetector 140. The modulated signal, the output signal of the photodetector 140 is the modulation frequency f _m to maximize is supplied to the semiconductor laser 110 while being finely adjusted. Laser light emitted from the semiconductor laser 110 is modulated by a modulation signal, and generates a first sideband wave W1 and a second sideband wave W2.

  The semiconductor laser 110, the wavelength selection device 120, the gas cell 130, and the photodetector 140 correspond to the light source 10, the wavelength selection unit 20, the gas cell 30, and the light detection unit 40 of FIG. The optical filter element 120a corresponds to the optical filter unit 20a in FIG. 1, and the optical filter characteristic control device 120b corresponds to the optical filter characteristic control unit 20b in FIG. The current driving circuit 150 and the modulation circuit 160 correspond to the control unit 50 in FIG.

  In the atomic oscillator 1 having such a configuration, the frequency difference between the first sideband wave W1 and the second sideband wave W2 of the first light L1 generated by the semiconductor laser 110 is the two ground levels of alkali metal atoms contained in the gas cell 130. Since the alkali metal atom does not cause the EIT phenomenon if it does not exactly match the frequency corresponding to the energy difference between the first sideband W1 and the irradiation light W2, the detection amount of the photodetector 140 is extremely sensitive. (Detection amount increases). Therefore, due to the feedback loop passing through the semiconductor laser 110, the wavelength selection device 120, the gas cell 130, the photodetector 140, and the modulation circuit 160, the frequency difference between the first sideband W1 and the second sideband W2 is 2 of alkali metal atoms. Feedback control is applied so as to match the frequency corresponding to the energy difference between the two ground levels very accurately. As a result, since the modulation frequency becomes a very stable frequency, the modulation signal can be used as the output signal (clock output) of the atomic oscillator 1.

  FIG. 5 is a cross-sectional view schematically showing main parts (semiconductor laser 110 and wavelength selection device 120) of the optical module 2.

  As shown in FIG. 5, the optical module 2 further includes a base 170. The semiconductor laser 110 and the wavelength selection device 120 are formed on the base 170. Thus, the semiconductor laser 110 and the wavelength selection device 120 are monolithically formed, so that the size of the device can be reduced.

  The material of the base 170 is a compound semiconductor. Specifically, the material of the base 170 is, for example, a group III-V semiconductor such as GaAs, InP, or GaN, or a group II-VI semiconductor such as ZnO or ZnSe. Here, a case where the material of the base 170 is a first conductivity type (for example, n-type) GaAs will be described.

  The semiconductor laser 110 is a surface emitting laser including a first semiconductor layer 112, an active layer 114, and a second semiconductor layer 116.

The first semiconductor layer 112 is formed on the base body 170. The first semiconductor layer 112 is, for example, a distributed Bragg reflection type in which n-type (first conductivity type) Al _0.9 Ga _0.1 As layers and n-type Al _0.15 Ga _0.85 As layers are alternately stacked. (DBR) A semiconductor mirror.

The active layer 114 is formed on the first semiconductor layer 112. The active layer 114 has, for example, a multiple quantum well (MQW) structure in which three quantum well structures each composed of a GaAs well layer and an Al _0.3 Ga _0.7 As barrier layer are stacked.

The second semiconductor layer 116 is, for example, a DBR semiconductor mirror in which p-type (second conductivity type) Al _0.15 Ga _0.85 As layers and p-type Al _0.9 Ga _0.1 As layers are alternately stacked. is there.

  The p-type second semiconductor layer 116, the active layer 114 that is not doped with impurities, and the n-type first semiconductor layer 112 form a pin diode.

  The electrode 118 of the semiconductor laser 110 is formed on the lower surface of the substrate 170. The electrode 118 is electrically connected to the first semiconductor layer 112 through the base body 170. The electrode 118 is one electrode for driving the semiconductor laser 110. The electrode 118 is a common electrode with the first electrode 128 of the optical filter characteristic control device 120b described later.

  The electrode 119 of the semiconductor laser 110 is formed on the upper surface of the second semiconductor layer 116. The electrode 119 is electrically connected to the second semiconductor layer 116. The electrode 119 is the other electrode for driving the semiconductor laser 110.

  When a forward voltage is applied to the pin diode by the electrodes 118 and 119, recombination of electrons and holes occurs in the active layer 114, and light emission due to the recombination occurs. Stimulated emission occurs when the generated light reciprocates between the second semiconductor layer 116 and the first semiconductor layer 112, and the light intensity is amplified. When the optical gain exceeds the optical loss, laser oscillation occurs, and laser light (first light L1) is emitted from the upper surface of the second semiconductor layer 116. The light L1 emitted from the semiconductor laser 110 is reflected by the prism 180 and enters the wavelength selection device 120 (optical filter element 120a). The prism 180 is an optical element for changing the traveling direction of the first light L1 and causing the first light L1 to enter the wavelength selection device 120 (the optical filter element 120a). In the present embodiment, the prism 180 is used to change the traveling direction of the first light L1, but if the traveling direction can be changed by reflecting the first light L1, another optical element is used. Also good. For example, a reflection mirror can be used.

  In the optical module 2, the optical filter element 120a is an etalon in the illustrated example. The optical filter element 120 a includes a first mirror 121, a second mirror 122, and a substrate 123 disposed between the first mirror 121 and the second mirror 122. The mirrors 121 and 122 are semi-transmissive mirrors that transmit part of the light incident on the mirror and reflect part of it.

  The optical filter element 120a transmits light in the predetermined wavelength range (first sideband W1 and second sideband W2) out of the incident light L1 due to the interference action between the mirrors 121 and 122. When light enters the optical filter element 120a, the interference between the mirrors 121 and 122 depends on the length D of the substrate 123 (the distance between the first mirror 121 and the second mirror 122) D and the refractive index of the substrate 123. The light having the selected wavelength is selectively transmitted through the optical filter element 120a. In the optical filter element 120a, the transmittance for the first sideband wave W1 and the second sideband wave W2 is large, and the transmittance for the fundamental wave F is small. As a result, the intensity of the fundamental wave F of the light L1 incident on the optical filter element 120a can be reduced or the fundamental wave F can be extinguished and emitted. Therefore, the optical filter element 120a can emit the second light L2 including only the first sideband W1 and the second sideband W2, for example.

The first mirror 121 and the second mirror 122 are opposed to each other with the substrate 123 interposed therebetween. The first mirror 121 and the second mirror 122 are made of, for example, a multilayer film in which Ta ₂ O ₅ films and SiO ₂ films are alternately stacked. The mirrors 121 and 122 have, for example, a layer structure in which three pairs of Ta ₂ O ₅ film and SiO ₂ film are formed. The reflectivity of the first mirror 121 and the second mirror 122 is, for example, 90%.

  The substrate 123 is formed on the base body 170. The substrate 123 includes a first layer 124, a second layer 125, and a third layer 126, which are sequentially formed from the base 170 side. The refractive index of the first layer 124 and the refractive index of the third layer 126 are smaller than the refractive index of the second layer 125. Therefore, the first layer 124 and the third layer 126 function as a cladding layer, and the second layer 125 functions as a core layer that propagates the first light L1. That is, the substrate 123 is an optical waveguide that propagates the first light L1. In the optical module 2, since the substrate 123 is an optical waveguide, the beam diameter of the light L2 emitted from the optical filter element 120a can be controlled, and the light L2 can be efficiently irradiated onto the gas cell 130. The material of the first layer 124 and the third layer 126 is, for example, AlGaAs, and the material of the second layer 125 is, for example, GaAs.

  The substrate 123 may not form an optical waveguide. The substrate 123 may be a single layer without having the plurality of layers 124, 125, and 126.

The material of the substrate 123 is not particularly limited, and may be a group III-V semiconductor such as GaAs, InP, or GaN, or a group II-VI semiconductor such as ZnO or ZnSe. For example, when GaAs is used as the material of the substrate 123, the length D of the substrate 123 is about 28.46 mm when the free spectral range is 9.2 GHz and the full width at half maximum is about 0.3 GHz. When general SiO ₂ is used as the material of the etalon substrate, the length of the substrate is about 70.66 mm. Thus, the length D of the substrate 123 can be shortened by using a compound semiconductor having a large refractive index as the material of the substrate 123.

The optical filter characteristic control device 120b includes a first electrode 128 and a second electrode 129 for supplying an electric field to the optical filter element 120a. The optical filter characteristic control device 120 b can supply an electric field to the substrate 123 by applying a voltage between the electrodes 128 and 129. As a result, an electro-optic effect occurs, the refractive index of the substrate 123 changes, and the wavelength selection characteristics of the optical filter element 120a can be changed. The first electrode 128 is located on the opposite side of the base 170 from the side where the optical filter element 120a is located. The first electrode 128 is formed below the base body 170. The first electrode 128 is a common electrode and the electrodes 118 of the semiconductor laser 110 described above. The second electrode 129 is located on the opposite side of the optical filter element 120a from the side on which the substrate 170 is located. The second electrode 129 is formed above the optical filter element 120a via the insulating layer 190. Electrodes 128, 129
By forming the insulating layer 190 therebetween, it is possible to prevent a current from flowing between the electrodes 128 and 129 to cause a voltage drop.

  Although the case where the optical filter element 120a is an etalon has been described here, the optical filter element 120a may be a fiber Bragg grating in which a periodic refractive index change is given to the core of the optical fiber. Since the fiber Bragg grating uses an optical fiber, it can be easily deformed, and the degree of freedom in design can be improved.

  Here, the wavelength selection characteristic of the optical filter element 120a is corrected using the electro-optic effect. For example, the thermo-optic effect (a phenomenon in which the refractive index of a substance with respect to light changes when heat is applied from the outside). May be used for correction. In this case, the optical module 2 may have a member for supplying heat to the optical filter element 120a.

  The optical module 2 and the atomic oscillator 1 have the following features, for example.

  According to the optical module 2, the wavelength selection device 120 can reduce the intensity of the fundamental wave F of the first light L1 or eliminate the fundamental wave F. Thereby, it can suppress or prevent that the fundamental wave F which does not contribute to an EIT phenomenon is irradiated to an alkali metal atom. Therefore, frequency fluctuation due to the AC Stark effect can be suppressed, and an oscillator with high frequency stability can be provided.

  According to the optical module 2, since the wavelength selection device 120 includes the optical filter characteristic control device 120b, the wavelength selection characteristic (optical filter) of the optical filter element 120a due to manufacturing error, environmental change (heat, light, etc.), etc. The shift in the wavelength range selected by the element can be corrected. Therefore, the wavelength selector 120 can select and emit the first sideband wave W1 and the second sideband wave W2 from the first light L1 with high accuracy.

  For example, when the optical filter element 120a is an etalon, the wavelength selection characteristic (the wavelength range selected by the optical filter element) of the optical filter element 120a depends on the length D of the substrate 123 and the refractive index of the substrate 123 shown in FIG. To do. However, in the manufacturing process of the optical filter element 120a, it is difficult to accurately control the length D of the substrate 123, and a manufacturing error may occur in the length D of the substrate 123. Even in such a case, since the optical module 2 includes the optical filter characteristic control device 120b, it is possible to correct the shift in the wavelength selection characteristic caused by the manufacturing error.

In the optical module 2, the optical filter element 120a is an etalon. Therefore, the optical filter element can have a simple configuration. The material of the substrate 123 of the optical filter element (etalon) 120a is a compound semiconductor. Therefore, the length D of the substrate 123 can be shortened compared to the case where the material of the substrate 123 is SiO ₂ , and the apparatus can be downsized.

  In the optical module 2, the optical filter characteristic control device 120b can change the wavelength selection characteristic of the optical filter element 120a by the electro-optical effect. Thereby, the wavelength selection characteristic of the optical filter element can be controlled with high accuracy and easily. Further, the optical filter characteristic control device 120 b includes a first electrode 128 and a second electrode 129 for applying a voltage to the substrate 123. Therefore, the optical filter characteristic control device 120b can be simplified.

  In the optical module 2, the wavelength selection device 120 and the semiconductor laser (light source) 110 are formed on the base 170. Thus, by forming the wavelength selection device 120 and the semiconductor laser 110 monolithically on the base 170, the size of the device can be reduced.

  In the optical module 2, as described above, the substrate 123 of the optical filter element (etalon) 120a is an optical waveguide. Therefore, the beam diameter of the light L2 emitted from the optical filter element 120a can be controlled, and the light L2 can be efficiently irradiated onto the gas cell 130.

  In the optical module 2, the semiconductor laser 110 is a surface emitting laser. Since the surface emitting laser has less current for generating a gain than the edge emitting laser, the power consumption can be reduced.

  The atomic oscillator 1 has an optical module 2. Therefore, as described above, the frequency stability can be increased.

  The above-described embodiment is an example, and the present invention is not limited to these.

  For example, although the semiconductor laser 110 shown in FIG. 5 described above is a surface emitting laser, the semiconductor laser may be an edge emitting laser. FIG. 6 is a cross-sectional view schematically showing the main part of an optical module 2D using an edge-emitting laser. In the following, in the optical module 2D shown in FIG. 6, the same reference numerals are given to the same configurations as those of the optical module 2 shown in FIG.

  The first semiconductor layer 112 is formed on the base body 170. As the first semiconductor layer 112, for example, a first conductivity type (for example, n-type) AlGaAs layer or the like can be used.

  The active layer 114 is formed on the first semiconductor layer 112. The active layer 114 has, for example, a multiple quantum well (MQW) structure in which three quantum well structures each composed of a GaAs well layer and an AlGaAs barrier layer are stacked.

  The second semiconductor layer 116 is formed on the active layer 114. As the second semiconductor layer 116, for example, a second conductivity type (for example, p-type) AlGaAs layer or the like can be used.

  For example, the p-type second semiconductor layer 116, the active layer 114 not doped with impurities, and the n-type first semiconductor layer 112 constitute a pin diode. Each of the second semiconductor layer 116 and the first semiconductor layer 112 is a layer having a larger forbidden band width and a smaller refractive index than the active layer 114. The active layer 114 has a function of amplifying light. The first semiconductor layer 112 and the second semiconductor layer 116 have a function of confining injected carriers (electrons and holes) and light with the active layer 114 interposed therebetween.

  In the semiconductor laser 110, when a forward bias voltage of a pin diode is applied between the electrodes 118 and 119, recombination of electrons and holes occurs in the active layer 114. This recombination causes light emission. Starting from this generated light, stimulated emission occurs in a chain, and the light intensity is amplified. When the optical gain exceeds the optical loss, laser oscillation occurs, laser light is generated, and laser light (first light L1) is emitted from the side surface of the active layer 114.

  According to the optical module 2D, by using an edge emitting laser as the semiconductor laser 110, it is possible to emit laser light perpendicular to the stacking direction of the layers 112, 114, and 116 of the semiconductor laser 110. Therefore, alignment between the semiconductor laser 110 and the wavelength selection device 120 can be performed by controlling the film thickness of each of the layers 112, 114, and 116. Therefore, the alignment accuracy between the semiconductor laser 110 and the wavelength selection device 120 can be improved. Furthermore, for example, an optical element such as a prism for making the laser beam incident on the wavelength selection device 120 becomes unnecessary.

  Although the embodiments of the present invention have been described in detail as described above, those skilled in the art will readily understand that many modifications are possible without substantially departing from the novel matters and effects of the present invention. Accordingly, all such modifications are intended to be included in the scope of the present invention.

1 atomic oscillator, 2 optical module, 10 light source, 20 wavelength selector,
20a optical filter unit, 20b optical filter characteristic control unit,
30 gas cell, 40 light detector, 50 controller, 110 semiconductor laser,
112 first semiconductor layer, 114 active layer, 116 second semiconductor layer,
118,119 electrodes, 120 wavelength selection device, 120a optical filter element,
120b optical filter characteristic control device, 121 first mirror, 122 second mirror,
123 substrate, 124 first layer, 125 second layer, 126 third layer,
128 first electrode, 129 second electrode, 130 gas cell,
140 photodetector, 150 current drive circuit, 160 modulation circuit, 170 substrate,
180 prism

Claims (8)

An optical module for an atomic oscillator using the quantum interference effect,
A surface emitting laser that emits light including a fundamental wave having a predetermined wavelength, and a sideband of the fundamental wave;
A wavelength selection unit that includes an incident unit on which light emitted from the surface-emitting laser is incident, and transmits the sideband among the light incident on the incident unit;
A gas cell in which an alkali metal gas is sealed and irradiated with light transmitted through the wavelength selection unit;
A light detector for detecting the intensity of light transmitted through the gas cell among the light irradiated to the gas cell;
A compound semiconductor substrate;
Including
The wavelength selector is
An optical filter that transmits light in a predetermined wavelength range;
An optical filter characteristic control unit that changes a wavelength range of light transmitted by the optical filter unit;
Have
The surface-emitting type laser is located on the substrate and includes an emission part that emits light,
The base includes a convex portion that protrudes with respect to a surface on which the surface-emitting laser is positioned on the base;
The optical filter portion is located on the convex portion,
The light emitted from the surface emitting laser is incident on the optical filter unit through a prism having a reflective surface,
The distance between the incident portion and the surface of the substrate on which the surface-emitting laser is located is greater than the distance between the emitting portion and the surface of the substrate on which the surface-emitting laser is located. Optical module for oscillator. The optical filter portion is an etalon,
The optical module for an atomic oscillator according to claim 1, wherein the optical filter characteristic control unit changes a wavelength range of light transmitted by the optical filter unit by an electro-optic effect. The optical filter section is
A first mirror and a second mirror that reflect the light incident on the wavelength selection unit and face each other;
A substrate disposed between the first mirror and the second mirror;
Have
The optical module for an atomic oscillator according to claim 2, wherein a material of the substrate is a compound semiconductor.   4. The optical module for an atomic oscillator according to claim 3, wherein the optical filter characteristic control unit includes a first electrode and a second electrode for applying a voltage to the substrate. The first electrode is located on the opposite side of the base on which the optical filter portion is located,
5. The optical module for an atomic oscillator according to claim 4, wherein the second electrode is located on a side opposite to a side where the base of the optical filter portion is located. The substrate of the optical filter portion has a first layer, a second layer, and a third layer laminated from the base side,
The refractive index of the first layer and the refractive index of the third layer are smaller than the refractive index of the second layer,
The optical module for an atomic oscillator according to claim 5, wherein the second layer propagates light emitted from the surface-emitting type laser.   An atomic oscillator comprising the optical module for an atomic oscillator according to any one of claims 1 to 6. An atomic oscillator that uses the quantum interference effect,
A surface emitting laser that emits light including a fundamental wave having a predetermined wavelength, and a sideband of the fundamental wave;
A wavelength selection unit that includes an incident unit on which light emitted from the surface-emitting laser is incident, and transmits the sideband among the light incident on the incident unit ;
A gas cell in which an alkali metal gas is sealed and irradiated with light transmitted through the wavelength selection unit;
A light detector for detecting the intensity of light transmitted through the gas cell among the light irradiated to the gas cell;
A compound semiconductor substrate;
Including
The wavelength selector is
An optical filter that transmits light in a predetermined wavelength range;
An optical filter characteristic control unit that changes a wavelength range of light transmitted by the optical filter unit;
Have
The surface-emitting type laser is located on the substrate and includes an emission part that emits light,
The base includes a convex portion that protrudes with respect to a surface on which the surface-emitting laser is positioned on the base;
The optical filter portion is located on the convex portion,
The light emitted from the surface emitting laser is incident on the optical filter unit through a prism having a reflective surface,
The distance between the incident portion and the surface of the substrate on which the surface-emitting laser is located is greater than the distance between the emitting portion and the surface of the substrate on which the surface-emitting laser is located. Oscillator.

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